Egr control device for an engine

ABSTRACT

An EGR control device for an engine comprising: an exhaust gas recirculating passage for connecting an exhaust pipe to an intake pipe of an internal combustion engine; an exhaust gas recirculating valve provided at the exhaust gas recirculating passage for controlling a quantity of exhaust gas which is recirculated to intake air of an engine; an oxygen sensor provided at an intake passage on the downstream side of a junction point of the recirculated gas and the intake air for detecting an oxygen concentration of the intake air mixed with the recirculated gas; an exhaust gas recirculating valve passage area controlling means for controlling a passage area of the exhaust gas recirculating valve; a running state detecting means for detecting a running state of the engine; a calculating means for calculating a first exhaust gas recirculation ratio based on an output of the oxygen sensor and for calculating a second exhaust gas recirculation ratio corresponding to a value detected by the running state detecting means; and a feedback controlling means for performing a feedback control which increases or decreases the passage area of the exhaust gas recirculation valve so that the first exhaust gas recirculation ratio agrees with the second exhaust gas recirculation ratio.

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

This invention relates to an EGR control device for an engine whichperforms a control wherein a portion of exhaust gas of an internalcombustion engine is recirculated to an intake pipe of the internalcombustion engine.

2. DISCUSSION OF BACKGROUND

Conventionally, an exhaust gas recirculation control device(hereinafter, EGR control device) performing a control of exhaust gasrecirculation (hereinafter, EGR) as a means of decreasing NO_(x) in theexhaust gas of the internal combustion engine, has widely been used.This EGR control device performs the control of EGR by an exhaustpressure control system utilizing a BPT (Back Pressure Transducer)valve.

Since the above-mentioned conventional EGR control device is constructedby using a BPT valve or the like, it can not detect an exhaust gasrecirculation quantity, that is, an EGR flow quantity. As a result, whenthe EGR flow quantity is increased by deterioration of the BPT valve orthe like, the drivability is worsened. Furthermore, when the EGR flowquantity is decreased, the NO_(x) component in the exhaust gas isincreased since the temperature of the engine is elevated.

Furthermore, even when this device is in an abnormal state bydeteriorations of parts of the EGR control device, this abnormality ishard to be detected.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an EGR controldevice for an engine capable of performing an accurate control inaccordance with a running state of the engine.

It is an object of the present invention to provide an EGR controldevice capable of performing an accurate control without worsening theexhaust gas irrespective of a deterioration of an exhaust gasrecirculation valve or the like.

It is an object of the present invention to provide an EGR controldevice for an engine capable of performing an accurate failuredetermination.

It is an object of the present invention to provide an EGR controldevice for an engine capable of reliably performing a failure detection.

According to the first aspect of the present invention there is providedan EGR control device for an engine comprising: an exhaust gasrecirculating passage for connecting an exhaust pipe to an intake pipeof an internal combustion engine; an exhaust gas recirculating valveprovided at the exhaust gas recirculating passage for controlling aquantity of exhaust gas which is recirculated to intake air of anengine; an oxygen sensor provided at an intake passage on the downstreamside of a junction point of the recirculated gas and the intake air fordetecting an oxygen concentration of the intake air mixed with therecirculated gas; an exhaust gas recirculating valve passage areacontrolling means for controlling a passage area of the exhaust gasrecirculating valve; a running state detecting means for detecting arunning state of the engine; a calculating means for calculating a firstexhaust gas recirculation ratio based on an output of the oxygen sensorand for calculating a second exhaust gas recirculation ratiocorresponding to a value detected by the running state detecting means;and a feedback controlling means for performing a feedback control whichincreases or decreases the passage area of the exhaust gas recirculationvalve so that the first exhaust gas recirculation ratio agrees with thesecond exhaust gas recirculation ratio.

According to the second aspect of the present invention there isprovided the EGR control device for an engine according to the firstaspect 1, further comprising the first memorizing means for memorizing afirst feedback correction value when the feedback-controlling isperformed.

According to the third aspect of the present invention there is providedthe EGR control device for an engine according to the first aspect,further comprising a first memorizing means for memorizing a firstfeedback correction value when the feedback controlling is performed;and a second memorizing means for memorizing a second feedbackcorrection value which is a further corrected value of the firstfeedback correction value when the feedback-controlling is performedutilizing the first feedback correction value memorized by the firstmemorizing means.

According to the fourth aspect of the present invention there isprovided the EGR control device for an engine according to the firstaspect 1, further comprising a failure determining means for determininga failure by detecting that the first exhaust gas recirculation ratiodisagrees with the second exhaust gas recirculation ratio.

According to the fifth aspect of the present invention there is providedthe EGR control device for an engine according to the second or thethird aspect, further comprising a failure detecting means for detectinga failure when the first feedback correction value or the secondfeedback correction value exceeds a first predetermined range or exceedsa second predetermined range obtained by parameters of the engine.

According to the sixth aspect of the present invention there is providedthe EGR control device for an engine according to the first aspect,further comprising a failure detecting device for detecting a failurewhen a difference between the first feedback correction value and thesecond feedback correction value exceeds a third predetermined range ora fourth predetermined range obtained by parameters of the engine.

In the first aspect of the invention, the device detects the oxygenconcentration in the intake air of the engine mixed with therecirculated exhaust gas by the oxygen sensor, and performs a feedbackcontrol by means of controlling the recirculated exhaust gas based onthe output signal of the oxygen sensor, thereby controlling it inaccordance with the running state of the engine.

In the second aspect of the invention, the device performs an accuratefeedback control by memorizing the feedback correction value in thefeedback control by the first memorizing means, without worsening theexhaust gas due to an age deterioration of the exhaust gas recirculationvalve of the like.

In the third aspect of the invention, the device performs the accuratefeedback control, when the device performs the feedback controlutilizing the first feedback correction value by memorizing the secondfeedback value which further corrects the first feedback value, in thesecond memorizing means, and without worsening the exhaust gas due to anage deterioration of the exhaust gas recirculation valve or the like.

In the fourth aspect of the invention, the device performs the failuredetermination by detecting that the first exhaust gas recirculationratio disagrees with the second exhaust gas recirculation ratio by thefailure determining means.

In the fifth aspect of the invention, the failure detecting meansperforms the failure detection by detecting that the first or the secondfeedback correction value exceeds a predetermined range, or exceeds apredetermined range obtained by the engine parameters.

In the sixth aspect of the invention, the failure detecting meansperforms the failure detection by detecting that a difference betweenthe first and the second exhaust gas recirculation ratios exceeds apredetermined range, or exceeds a range obtained from the engineparameters.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a block diagram showing an embodiment of an EGR controldevice;

FIG. 2 is a block diagram showing an electronic control unit of theembodiment in FIG. 1;

FIG. 3 is a construction diagram of an oxygen sensor and an oxygenconcentration detecting device in the embodiment in FIG. 1;

FIG. 4 is a characteristic diagram showing a relationship between theoxygen concentration and an exhaust gas recirculation ratio forexplaining the embodiment in FIG. 1;

FIG. 5 is a characteristic diagram showing a relationship between a pumpcurrent and an EGR ratio for explaining the embodiment in FIG. 1;

FIG. 6 is a flowchart showing a treatment of a main routine forexplaining the operation of the embodiment in FIG. 1;

FIG. 7 is a flowchart of an actual EGR ratio detecting treatment in theembodiment in FIG. 1;

FIG. 8 is a flowchart of an EGR control treatment in the embodiment inFIG. 1;

FIG. 9 is a characteristic diagram of (target EGR ratio--actual EGRratio) versus a control gain for explaining the embodiment in FIG. 1;

FIG. 10 is a characteristic diagram showing a relationship between anEGR control value versus a control duty for explaining the embodiment inFIG. 1;

FIG. 11 is an explanatory diagram showing the definition of the controlduty for explaining the embodiment in FIG. 1;

FIG. 12 is a flowchart of a main routine of a second embodiment of anEGR control device for an engine according to the present invention;

FIG. 13 is a flowchart of an EGR control treatment of the secondembodiment;

FIG. 14 is a flowchart of a main routine of a third embodiment of an EGRcontrol device for an engine according to the present invention;

FIG. 15 is a flowchart of the third embodiment of a failure determiningtreatment; and

FIG. 16 is a flowchart of another embodiment of a failure determiningtreatment in an EGR control device for an engine according to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, explanation will be given to an embodiment of an EGR controldevice for an engine of this invention referring to the drawings.

FIG. 1 is a block diagram showing an embodiment thereof. In FIG. 1, areference numeral 1 designates an engine, 2, an air cleaner, 3, anintake pipe, 4, an intake manifold, 5, an injector, 6, a pressuresensor, 7, a throttle valve, 8, a throttle opening degree sensor, 11, anexhaust gas recirculating valve (hereinafter recirculating valve), 12, apassage area control actuator (hereinafter, EGR solenoid), 13, anignition coil, 14, an ignitor, 15, an exhaust pipe, 17, a watertemperature sensor, 20, a battery, 21, an ignition key switch, 22, anelectronic control unit, 23, an alarming lamp, 24, an oxygen sensor and25, an oxygen concentration detecting device.

In FIG. 1, the pressure sensor 6 is a semiconductor type pressure sensorwhich detects an intake air pressure for measuring an air quantitysucked to the engine 1 through the intake pipe 3 and the intake manifold4.

Each of the injectors 5 is provided to the intake manifold 4 at adjacentto each exit to the respective cylinders, and supplies fuel to theengine 1 by control of the electronic control unit 22. The watertemperature sensor 17 is a thermistor type sensor which detects acooling water temperature of the engine 1.

The ignition coil 13 performs ignition by a signal from the ignitor 14,and transmits the generated ignition signal to the electronic controlunit 22.

The recirculating valve 11 is a vacuum servo type valve installed at anexhaust gas recirculation passage which connects the intake pipe 3 withthe exhaust pipe 15.

The EGR solenoid 12 is connected to a diaphragm chamber of therecirculating valve 11 and the intake pipe 3, and controls a negativepressure of the diaphragm chamber of the recirculating valve 11 by asignal from the electronic control unit 22. A passage area of therecirculating valve 11 becomes variable by the negative pressure of thediaphragm chamber.

Next, the electronic control unit 22 controls the passage area of therecirculating valve 11 by receiving respective signals of the pressuresensor 6, the throttle opening degree sensor 8, the ignition coil 13 andthe water temperature sensor 17, as signals of a running state detectingmeans.

Accordingly, the electronic control unit 22 controls to drive the EGRsolenoid 12 by obtaining a control quantity of the EGR solenoid forcontrolling the EGR quantity and has a function as a feedback controlmeans.

The oxygen concentration sensor 24 is provided on the downstream side ofa junction point of the recirculation (EGR) gas and the intake air, anddetects an oxygen concentration of the intake air mixed with the EGRgas.

The electronic control unit 22 receives an output of the oxygen sensor24 through the oxygen concentration detecting device 25 and has afunction as a calculating means for calculating an EGR ratio.

FIG. 2 is a detailed block diagram of the electronic control unit 22. InFIG. 2, a reference numeral 100 designates a microcomputer, which iscomposed of a function of a calculating means for calculating a controlquantity of the EGR solenoid or the like in accordance with apredetermined program, a CPU 200 having a failure determining function,a free running counter 201 for measuring a revolution period of theengine 1, a timer 202 for timewisely measuring a duty ratio of a drivingsignal applied to the EGR solenoid, an A/D-convertor 203 for convertingan analog input signal to a digital signal, a RAM 205 utilized as a workmemory which memorizes various feedback correction values or the like, aROM 206 wherein programs are memorized, an output port 207 foroutputting the driving signals and a common bus 208.

A numeral 101 designates a first input interface circuit, which outputsa primary side signal shaped of its waveform of the ignition coil 13, tothe microcomputer 100 as an interruption signal.

When the interruption signal is generated, the CPU 200 reads a value ofthe counter 201, calculates a period of the engine revolution number bya difference between the current read value and a preceding read valueand memorizes it in the RAM 205.

A numeral 102 designates a second input interface circuit, whichreceives respective signals of the pressure sensor 6, the throttleopening degree sensor 8, the water temperature sensor 17 and the like,and output them to the A/D convertor 203.

A numeral 104 designates a first output interface circuit, whichamplifies a driving output from the output port 207 and output it to theEGR solenoid 12.

An output of the oxygen sensor 24 is inputted to the second inputinterface 102 through the oxygen concentration detecting device 25. Anumeral 107 designates a second output interface, which controls areference voltage (V_(R)) of the oxygen concentration detecting device25.

A numeral 103 designates a third input interface circuit, of whichoutput is transmitted to the common bus 208 through the input port 204.

The battery 20 and the ignition switch 21 are connected to a power lineand the ground in series. A junction point of the power line and theignition switch 21 is connected to a first power source circuit 105. Aconnection point of the battery 20 and the ignition switch 21 isconnected to a second power source circuit 106.

The first power source circuit 105 and the second power source circuit106 supply power to the microcomputer 100.

FIG. 3 is a detailed construction diagram of the oxygen sensor 24 andthe oxygen concentration detecting device 25. The construction of theoxygen sensor 24 is already publicly known (Japanese Unexamined PatentPublication No. 138263/1985). A brief explanation will be given todetection of the EGR ratio utilizing the oxygen sensor 24.

In FIG. 3, numerals 33 and 37 designate solid electrolytes (Zirconia),to respective both sides of which platinum electrodes 34 and 35, and 38and 39, are provided.

An oxygen pump 32 is constructed by the solid electrolyte 33 and theplatinum electrodes 34 and 35. An oxygen concentration cell 30 isconstructed by the solid electrolyte 37 and the platinum electrodes 38and 39. Both are opposedly provided interposing a very small clearance"d" of about 0.1 mm.

In this way, the oxygen sensor 24 is constructed by the oxygenconcentration cell 30, the oxygen pump 32 and a supporting element 31.The oxygen sensor 24 is supported by the supporting element 31 and isattached to the intake manifold 4.

The oxygen concentration detecting device 25 is constructed as in FIG.3, drives a transistor T_(R) by an output of a differential integralamplifier composed of a resistance R₁, a condenser C and a calculationamplifier A, controls a pump current I_(P) flowing in the platinumelectrodes 34 and 35 of the oxygen pump 32, and has a voltage "e"generated between the platinum electrodes 38 and 39 of the oxygenconcentration cell 30, agree with a difference voltage V_(R), as aninput terminal voltage B.

A resistance R_(O) is connected to an emitter of the transistor T_(R)and the platinum electrode 34, whereby an output voltage correspondingto the pump current I_(P) which is in proportion to the oxygenconcentration in the intake air of the intake manifold 4 mixed with theexhaust gas, is obtained.

FIG. 4 is a characteristic diagram showing a relationship between theoxygen concentration of the intake air mixed with the recirculatedexhaust gas, and a mixing ratio (EGR ratio) of the exhaust gas.According to FIG. 4, the oxygen concentration changes in inverseproportion to the exhaust gas EGR ratio.

FIG. 5 is a characteristic diagram showing a relationship between thepump current I_(P) and the EGR ratio. This characteristic diagram showsa change of the pump current when the EGR ratio of the engine 1 ischanged, while maintaining the reference voltage V_(R) at 50 mV.According to the characteristic diagram 5, the pump current I_(P)changes in inverse proportion to the EGR ratio.

Next, explanation will be given to the operation of the CPU 200 of theabove constructed EGR control device referring to flowcharts. FIG. 6shows a treatment of a main routine.

In step 400, the operation performs other control treatments. In step401, the operation performs an actual EGR ratio detecting treatment fordetecting an actual EGR ratio in a running state of the engine. In step402, the operation performs an EGR control treatment based on the actualEGR ratio detected in step 401, and goes back to step 400.

Next, explanation will be given to the actual EGR ratio detectingtreatment of FIG. 7. In step 410, the operation reads the pump currentI_(P) by A/D-converting both terminals voltage of the resistance R_(o)in the oxygen concentration detecting device 25. In step 411, theoperation obtains the actual EGR ratio P_(EGR) by the relationshipbetween the pump current I_(P) and the EGR ratio shown in FIG. 5. Instep 412, the operation sets an actual EGR ratio detection flag.

When the actual EGR ratio P_(EGR) is calculated, the actual EGR controltreatment shown in FIG. 8 is performed, based on the actual EGR ratio.In step 450, the operation detects an engine revolution number Ne, andsuccessively detects an intake manifold pressure Pb in step 451.

Next, the operation determines an EGR operational range based on theengine revolution number Ne and the intake manifold pressure Pb in step452. In step 453, the operation determines whether the engine is in theEGR operational range. When the engine is in the EGR operational rangeas a result of this determination, the operation calculates a target EGRratio (a second EGR ratio) based on the engine revolution number Ne andthe intake manifold pressure Pb in step 454. In step 455, the operationcalculates a basic EGR control quantity K_(BASE) corresponding to thetarget EGR ratio T_(EGR).

In step 456, the Operation determines whether the actual EGR ratiodetection flag is set. When it is set, the operation resets the actualEGR ratio detection flag in step 457. In step 458, the operationcalculates a control gain ΔK_(EGR) by a value of the target EGR ratioT_(EGR) subtracted by the actual EGR ratio P_(EGR), based on a graphshown in FIG. 9.

FIG. 9 is a graph showing a characteristic of the control gain ΔK_(EGR).A value of the target EGR ratio T_(EGR) subtracted by the actual EGRratio P_(EGR) is denoted in abscissa, and a value of the control gainΔK_(EGR) corresponding to the abscissa value, in ordinate, respectively.

In step 459, the operation calculates an EGR control correction valuewherein the control gain ΔK_(EGR) is added with the EGR controlcorrection value K_(EGR) before calculation.

In step 460, the operation calculates an EGR control value K of the EGRcontrol correction value obtained in step 459, added with the basiccontrol quantity K_(BASE). In step 461, the operation Calculates acontrol duty D_(EGR) from the obtained EGR control value K, based on agraph showing a relationship between the EGR control value K and thecontrol duty D shown in FIG. 10. In step 462, the operation drives theEGR solenoid 12 based on the control duty D_(EGR).

In this way, a deviation between the target EGR ratio T_(EGR) and theactual EGR ratio P_(EGR) is nullified and the target EGR ratio T_(EGR)agrees With the actual EGR ratio P_(EGR).

FIG. 11 is an explanatory diagram showing the definition of the controlduty D. Assuming ON time as T_(ON), and a period as T, the control dutyD is shown by the following equation.

    D=(T.sub.ON /T)×100 (%)

Furthermore, when the engine is out of the EGR operational range, forinstance, in an idling state, and when the operation determines as "N"in step 453, the operation sets the EGR control quantity K as 0 in step463, to nullify the EGR control quantity, and calculates the controlduty D_(EGR) from the EGR control quantity of 0, in step 461.

When the actual EGR ratio detection flag is not set, and the operationdetermines as "N" in step 456, the operation proceeds to step 460. Instep 460, the operation calculates the EGR control quantity value basedon the EGR control correction value K_(EGR) which is not added With thecontrol gain ΔK_(EGR), and the basic EGR control quantity K_(BASE),based on which successive treatments are performed in step 461 and soon.

FIGS. 12 and 13 are flowcharts showing the operation of a secondembodiment of the EGR control device of this invention. Explanation willbe given to the flowchart of FIG. 12.

In step 500, the operation determines whether first power is ON afterthe battery 20 is provided. The operation determines it by detecting anoutput voltage of the second power source circuit 106 connected to thebattery 20 is changed from a low voltage value to a high voltage value.

When the operation determines as "Y" as a result of the determination,the operation sets the EGR control correction value K_(EGR) as 0 in step501. In step 502, the operation performs the other control treatments.In step 503, the operation performs a steady state running determiningtreatment. In step 504, the operation performs an actual EGR ratiodetecting treatment. In step 505, the operation determines the EGRcontrol treatment.

Furthermore, when the operation determines that the first power is ONafter the battery 20 is provided in step 500, as "N", that is, when thebattery 20 is already provided, and only the ignition switch 21 isturned on, the operation does not set the EGR control correction valueK_(EGR) as 0, and utilizes the EGR control correction value K_(EGR)already memorized in the RAM 205 in the succeeding treatments of step502 and so on.

Next, explanation will be given to a flowchart of FIG. 13. The treatmentin steps 550 through 559 in this flowchart, is the same as that in steps450 through 459 of the flowchart in FIG. 8, and a detailed explanationwill be omitted.

That is, in steps 550 through 559, the operation calculates the EGRcontrol correction value K_(EGR) in the EGR operational range. In step560, the operation memorizes the calculated EGR control correction valueK_(EGR). In step 561, the operation calculates the EGR control value Kbased on the EGR control correction value obtained in step 559, addedwith the basic control quantity K_(BASE). In step 562, the operationcalculates the control duty D_(EGR) based on the obtained EGR controlvalue K. In step 563, the Operation drives the EGR solenoid 12 based onthe control duty D_(EGR).

In this way, the operation memorizes the EGR control correction valueK_(EGR) when it is calculated. In case that, when power is ON in thisdevice, it is not the first power ON after the battery 20 is provided,the operation utilizes the memorized EGR control correction valueK_(EGR) as a correction value before calculation. Accordingly, anaccurate EGR control can be performed instantly after the ignitionswitch 21 is turned ON. When the engine is out of the EGR operationalrange in step 553, the operation sets the EGR control value K as 0 instep 564, and calculates the control duty D_(EGR) from the EGR controlvalue of 0, in step 562.

FIGS. 14 through 16 are flowcharts showing the operation of a thirdembodiment of the EGR control device of this invention. First,explanation will be given to a flowchart of FIG. 14.

In steps 600 through 603, in FIG. 14, the operation successivelyperforms the other control treatment, the actual EGR ratio detectingtreatment and the EGR control treatment, as in steps 400 through 402 ofthe flowchart of FIG. 6.

After the EGR control treatment in step 603, is performed, the operationperforms a failure determining treatment for determining a failure ofthe device in step 604, and returns to step 600.

Next, explanation will be given to a flowchart of FIG. 15 concerning adetailed failure determining treatment of this device. In FIG. 15, theoperation performs a determination whether the EGR control correctionvalue K_(EGR) is smaller than a predetermined intolerable value B, as aresult of for instance, an exhaust gas test, in step 650. When it islarger than the predetermined value B, the operation performs adetermination whether the EGR control correction value K_(EGR) is largerthan a predetermined intolerable value C, as a result of for instance,the exhaust gas test in step 651. When it is smaller than thepredetermined value C, the operation determines the EGR control deviceas normal and sets a flag meaning the normality in step 652, and turnsoff the alarming lamp 23 in step 653.

Furthermore, when the EGR control correction value K_(EGR) is smallerthan the predetermined value B, and the Operation determines as "Y" instep 650, or, when the EGR control correction value K_(EGR) is largerthan the predetermined value C, and the operation determines as "Y" instep 651, the operation determines the EGR control device as abnormal instep 654 and sets a flag meaning the abnormality, and turns on thealarming lamp 23 in step 655.

Accordingly, this invention determines that the EGR control device is infailure when the target EGR ratio T_(EGR) disagrees with the actual EGRratio P_(EGR).

Next, explanation Will be given to another embodiment of the failuredetermination of the EGR control device based on a flowchart of FIG. 16.In step 700 of FIG. 16, the operation determines an absolute value of avalue of the target EGR ratio T_(EGR) subtracted by the actual EGR ratioP_(EGR), as E. In step 701, the operation determines whether theabsolute value E is larger than a predetermined intolerable value F as aresult of for instance, an exhaust gas test.

When the absolute value E is smaller than the predetermined value F, theoperation determines the EGR control device as normal in step 702, andsets a flag meaning the normality, and turns off the alarming lamp 23 instep 703.

Furthermore, when the absolute value E is larger than the predeterminedvalue F, and the operation determines as "Y" in step 701, the operationdetermines the EGR control device as abnormal in step 704, sets a flagmeaning the abnormality, and turns on the alarming lamp 23 in step 705.

In this embodiment, a comparison is made between the absolute value Eand the predetermined value F showing a deviation of the target EGRratio T_(EGR) deviated from the actual EGR ratio P_(EGR), and as aresult, the Operation instantly determines the failure of the device.However, the operation may determine the failure of the device after thelarge-or-small relationship of the absolute value E with respect to thepredetermined value F continues for a constant time, by introducing atime counting means of the timer 202.

The operation may determine the failure by using the counter 201, and bycounting number of time wherein the device is determined as abnormal inthe failure determination of steps 650 and 651 in FIG. 15, and when theabnormality continues for a predetermined number of times.

As stated above, according to the first aspect of the invention, sincethe passage area of the recirculating valve is controlled to increase ordecrease so that the first EGR ratio agrees with the second EGR ratio,an accurate recirculation control of the exhaust gas in accordance withthe various running states, can be performed.

According to the second aspect of the invention, since it is constructedto memorize the first feedback correction value to the first memorizingmeans when the feedback control is performed so that the deviationbetween the first EGR ratio and the second EGR ratio is nullified, itcan perform an accurate control without deteriorating the exhaust gasdue to the age deterioration of the EGR valve or the like.

According to the third aspect of the invention, since it is constructedto memorize the second feedback correction value to the secondmemorizing means which further corrects the first feedback correctionvalue when the feedback control is performed by utilizing the firstfeedback correction value memorized in the first memorizing means, theinvention has an effect of performing the control more accuratelywithout worsening the exhaust gas due to the age deterioration of theEGR valve or the like.

According to the fourth aspect of the invention, since it is constructedto determine the failure when the first EGR ratio disagrees with thesecond EGR ratio by the failure determining means, the invention has aneffect wherein the failure of the device can directly and accurately bedetected.

According to the fifth aspect of the invention, since it is constructedto detect the failure when the first or the second feedback correctionvalue exceeds a predetermined range or when a difference between theactual EGR ratio and the target EGR ratio obtained by parameters of theengine, exceeds a predetermined range, the failure of the device candirectly and accurately be detected as above.

According to the sixth aspect of the invention, since the failure isdetermined by the failure detecting means when a difference between thefirst EGR ratio and the second EGR ratio exceeds a predetermined range,or when the EGR ratio obtained by parameters of the engine exceeds apredetermined range, the invention has an effect wherein the failure ofthe device can directly and accurately be detected.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed is:
 1. An EGR control device for an engine comprising:anexhaust gas recirculating passage for connecting an exhaust pipe to anintake pipe of an internal combustion engine; an exhaust gasrecirculating valve provided at the exhaust gas recirculating passagefor controlling a quantity of exhaust gas which is recirculated tointake air of an engine; an oxygen sensor provided at an intake passageon the downstream side of a junction point of the recirculated gas andthe intake air for detecting an oxygen concentration of the intake airmixed with the recirculated gas; an exhaust gas recirculating valvepassage area controlling means for controlling a passage area of theexhaust gas recirculating valve; a running state detecting means fordetecting a running state of the engine; a calculating means forcalculating a first exhaust gas recirculation ratio based on an outputof the oxygen sensor and for calculating a second exhaust gasrecirculation ratio corresponding to a value detected by the runningstate detecting means; and a feedback controlling means for performing afeedback control which increases or decreases the passage area of theexhaust gas recirculation valve so that the first exhaust gasrecirculation ratio agrees with the second exhaust gas recirculationratio.
 2. The EGR control device for an engine according to claim 1,further comprising a first memorizing means for memorizing the firstfeedback correction value when the feedback controlling is performed. 3.The EGR control device for an engine according to claim 1, furthercomprising a first memorizing means for memorizing a first feedbackcorrection value when the feedback controlling is performed; and asecond memorizing means for memorizing a second feedback correctionvalue which is a further corrected value of the first feedbackcorrection value when the feedback controlling is performed utilizingthe first feedback correction value memorized by the first memorizingmeans.
 4. The EGR control device for an engine according to claim 1,further comprising a failure determining means for determining a failureby detecting that the first exhaust gas recirculation ratio disagreeswith the second exhaust gas recirculation ratio.
 5. The EGR controldevice for an engine according to claim 2 or claim 3, further comprisinga failure detecting means for detecting a failure when the firstfeedback correction value or the second feedback correction valueexceeds a first predetermined range or exceeds a second predeterminedrange obtained by parameters of the engine.
 6. The EGR control devicefor an engine according to claim 1, further comprising a failuredetecting device for detecting a failure when a difference between thefirst feedback correction value and the second feedback correction valueexceeds a third predetermined range or a fourth predetermined rangeobtained by parameters of the engine.